Apparatus for demagnetizing a magnetic recording-reproducing head



Jan. 23, 1951 s BEGUM 2,538,893

APPARATUS FOR DEMAGNETIZING A MAGNETIC RECORDING-REPRODUCING HEAD FiledMarch 25, 1947 3 Sheets-Sheet l J F0 RWARD FAST REV" N 0 RECORD STOPFORWARD CLEAR INVENTOR S..J.BEGUN ATTORNEY Jan. 23, 1951 5, BEGUM2,538,893

APPARATUS FOR DEMAGNETIZING A MAGNETIC RECORDINGREPRODUCING HEAD FiledMarch 25, 1947 3 Sheets-Sheet 2 /99 V I 75 OsciI/afor r E1953 agINVENTOR $.J. BEGUN ATTORNEY FORWARD REELiNG Jan. 23, 1951 5. J. BEGUN2,538,893

APPARATUS FOR DEMAGNETIZING A MAGNETIC RECORDING-REPRODUCING HEAD Filedllarch 25, 194'? r r 3 SheetS -Sheet 5 REVHNNNG INVENTOR $.J. BEGUNATTORNEY relented Jan. 23, 1951 FFICE APPARATUS FOR DEMAGNETIZING A MAG-NETIC RECORDING-REPRODUCING HEAD Semi Joseph Begun, Cleveland Heights,Ohio, as-

signor to The Brush Development Company, Cleveland, Ohio, a corporationof Ohio Application March 25, 1947, Serial No. 737,163 1: Claims. (Cl.ire-100.2)

This invention relates to magnetic recording and reproducing and isespecially directed to improvements for avoiding unsatisfactoryperformance in what may otherwise be a high quality transducingapparatus.

Such apparatus contain magnetic record transducing heads having one ormore magnetic cores usually consisting of highly permeable metalprovided'with a non-magnetic gap along which successive elementalportions of an elongated permanently magnetizable record track are movedinto magnetic linkage with the core. The cores are also usually providedwith wind-' ings carrying electric currents corresponding to themagnetic flux variations in the core. Magnetic erasing, recording orreproducing may be effected with such heads, and to diminish eddycurrent losses their cores are generally made of one or more thicknessesof thin magnetic sheetmetal lamination.

It has been found that under certain conditions of operation, the noiselevel of a playback increases to such an extent as to be extremelyobjectionable, in spite of previously satisfactory performance. This waspreviously considered as caused by the retention of permanent magnetismby the cores due to the remanence of some of the signal flux. Theremanence has been ascribed to the difllculty of completely magneticallyneutral zing some of the core materials which in the case of recordingcores, for example, could be an alloy, such as silicon steel, having avery high magnetic saturation. Other magnetic core materials, such asthose useful for magnetic reproduction, could be selected from the highinitial permeab lity a loys, such as the commercially available alloy ofiron containing 76% nickel, 6% copper and 1.5% chromium and althoughthese materia's may be made to exhibit substantially zero remanence,they are known to be highly susceptible to mechanical strain whichrenders them permanently magnetizable.

It has @now been discovered, however, that the noise level may rise toobjectionable heights as a result of minor disturbances that aresometimes ignored. For example, a completely demagnetized core may beginto introduce noise after exposure to minor electric or magnetic fieldsof force leaking from around electric motors, relays or wiring. Forexample, the mere performance of one or more reeling operations, such asa rewinding, will sometimes obiectionably increase the noise level eventhough the cores are completely demagnetized before the reeling.

This is apparently due to the fact that in spite of the greatest care inpreparation, a magnetic core when mounted in a transducer almostinvariably develops permanently magnetizable characteristics.

Even expedients that tend to decrease mechanical straining of the corehave not been too successful in avoiding the difliculty. For example,using more and thinner core laminations so as to cause mechanicalstresses to be more readily absorbed by slippage between laminationsrather than the yielding of a lamination, improves the situation but isnot a complete prevention. The very act of fastening the core in placemay strain the core sufliciently for developing the undesiredcharacteristics.

The complete shielding of the cores is extremely troublesome if notimpossible and may not eliminate the difliculties inasmuch as the corewindings are connected to an assembly of switches which have contactsthat cannot easily be shielded or suitably spaced.

Among the objects of this invention is novel magnetic transducingapparatus including means for demagnetizing the magnetic cores wheneverdesired.

Another object of the invention is novel transducing apparatuscontaining means for automatically demagnetizing the magnetic coreswhenever the apparatus is set for a predetermined operating conditionwithout adversely affecting the magnetic condition of the record track.

The foregoing and other objects of the invention will be best understoodfrom the following description of exem'plifications thereof, referencebeing bad to the accompanying drawings wherein:

Fig. 1 is a diagrammatic representation of a novel magnetic transducingapparatus embodying the invention;

Fig. 2 indicates a modified construction of a portion of the apparatusof Fig; 1; and

Fig. 3 illustrates diagrammatically another transducing apparatusaccording to a modified form of the invention.

In Fig. 1 there is shown the essential elements of a magnetic transducerin which an elongated magnetic record track}! may be reeled in forwarddirection, as indicated by the arrow BI-F, from a supply reel 20 to atake-up reel 2| along a guide path between two guide rollers 26. Alongthe guide path between the guide rollers 26 there is positioned atransducing head 35 containing two magnetic cores 31, 38 and a capstanroller 40. Each magnetic core includes an almost completely closedmagnetic circuit extending substantially entirely on one side of theguide path and having the circuit leg facing the guide path shaped toprovide a smooth record track guiding surface 42 as well as anon-magnetic gap II by which the cores are magnetically linked withsuccessive elemental portions of the moving record track. windings 44are interlinked with the cores so as to carry electrical currentscorresponding to the magnetic flux in the cores.

The guide faces 42 of the cores are shown as displaced to one side ofthe straight line path represented by the dash-double-dot line 3I-S sothat when undeflected the record track will move along the path IS andbe so far removed from the magnetic cores as to be beyond their zone ofmagnetic linkage.

Opposite the transducing head and capstan roller 40 is positioned adeflecting structure generally indicated as 46, and equipped withdeflecting guides 48 and 50 and pressing pads 52. The deflecting guidesand pads are movable in a direction generally transverse to the guidepath as by mounting the guide 48 and the pads 52 on a slide held betweenslide guides 04 and linked as by the cross-arm 56 with the deflectingguide 50.

The guides 48 and 50 may be in the form of rollers rotatably held ontheir arms and the pressing pads 52 may be resiliently mounted on slide40 so as to be urged toward the magnetic cores as by biasing the padholders 53. The guide 00 may be arranged for Operation with the slide asby the pivotal mounting 58- and the anchor pin 00. The guide 50 may alsobe resiliently urged as by including in the arm a thin springy sectionas shown.

The guides, particularly guide 50 which is used to press the recordtrack against thecapstan roller 40, may also be mounted so as to beautomatically adjustably tilted to insure the substantially uniformpressing of the entire height of the guide 50, for example, against thecapstan roller. Without this self-adjusting feature the record track mayhave atendency to ride up or down ports to the arms by yieldablespring-like mem- 4 4 out No. 2,535,486, granted December 26, 1950, inwhich suitable details of these features are given.

The construction of Fig. 1 also provides for rewinding a record trackand for reeling it forward without transduclng. All the operations areshown as effected by a control assembly 01 including a set of push rods0|, 02, 03, 04, Hollerated by buttons IOI, I02, I00, I04 and I0,respectively. The rods are shown as interlinked in the conventional formof push button assembly in which inward actuation of a button from anouter retracted position in which the buttons I02, I03, I04 and I05 areillustrated to an inward position in which the button IN is illustrated,causes the inwardly moved button ,become latched in place and at thesame time gs about outward retraction of any other button or buttonsthat may have been inwardly latched.

As shown the button operation is provided by slidably mounting the pushrods in a frame 00,

outwardly biasing the rods as by springs 00, stops 80 limitingthe=outward travel. Each of the rods which the latch plate is returnedto the left by its bias to engage the outer face of the cam I01 and holdthe button in. At the same time the sliding of the latch plate to theright removes any obstruction from behind inwardly positioned camprojections so that the outward bias 00 of any push rod previously heldin inward position.

bers which permit one or both ends of the sup- I ports to tilttransversely to the guide path. The pivot 50 may also betiltably'self-adiusting.

The deflecting structure 40 operates to bring the record track from thestraight line path I I-S part of the present invention and any recordtrack impelling arrangement may be used. The specific details of asuitable construction are shown in the Williams application, Serial No.

732,970 filed March 7, 1947.

The pads 02 serve to press the record track into stable magnetic linkagewith the magnetic cores as disclosed and claimed in the Dankapplication,

581111 No. 690,878, filed August 16, 1 946, new Pat- The constructionillustrated permits the simultaneous inward positioning of more than onebutton as for example when recordings are to be made. For this operationthe two push buttons labelled Forward and Record may be simultaneouslyheld in their inward positions until the latch plate IIII slides inbehind their cams.

I The control assembly also includes'means, not

shown, for effecting the various reeling opera-' tions asindicated bythe legends on the push buttons. The Forward button I0l controlsconstant speed reeling such as is required for transduclng; the Fastforward button I02 may be used for forward reeling without transduclng,as where it is desired to reach a specific portion of the recording, andsuch reeling need not be made at a constant speed and may be done athigh speed to save time.

The Rewind push button I00 controls the reversed reeling of the recordtrack from the takeup reel 2i back to the supply reel 20, which may alsobe at high speed and the Record push button I04 establishes the natureof a transduclng operation. When the Record push button is in itsoutward restricted position the magnetic head is connected for playingback recordings and when this button is in its inward. operatedposition, the magnetic cores are connected for making recordings on -therecord track.

The Stop-clear button I0! is used to stop any reeling and to clarify theplayback in accordance with the invention, as indicated below.

The push rodsmay be arranged for coactlng with electrical or mechanicalswitching or drive control arrangements, not shown, for applying recordtrack impelling forces to the supply reel, take-up reel or capstanroller or to any desired combination of reel and roller as shown, forexample, in the above mentioned Dank application.

One form of transducing circuit suitable for use with the invention isillustrated in the figure and includes a switch group I20 for connectingthe windings 44 of the cores with the various elements of amplifying andsignal transducing units. With the Record push button retracted, theswitch group I20 by one of its switches I33 connects the windings of therecording-reproducing core 38 to the input of preliminary amplifier I0,the output of which through switch I34 is fed through auxiliary playbackamplifier I2 to a signal reproducer such as loudspeaker 14. Wheninwardly actuated the Record push rod through a suitable linkageoperates the switch group I20 so that switch I33 connects the input ofthe preliminary amplifier I to a source of signals such as themicrophone 69 and switch I34 feeds the amplified output to an auxiliaryrecording amplifier II. The output of amplifier II is fed through switchI32. to the windings of the recording-reproducing head 30. As'illustrated, tine apparatus employs A. C. biasing for diminishingdistortion during recording, the high frequency oscillator I6 feeding asmall biasing current through switch I 3| and the condenser 18 in amanner well known in the art. The A. C. current limiting of thecondenser permits most of the oscillator output to pass through thewindings of the core 31 to furnish a high frequency magnetic erasingfield at its gap H.

By suitable mechanical linkage such as that shown the Forward push rodis arranged to move the deflector assembly into the operated positionshown wherein it brings the record track 3| into the transducing path3IT, into stable magnetic linkage with the cores 31, 30 and intosuitable frictional driving engagement with the capstan roller 40.

The details of -the amplifying, transducing, record track and reelingunits as well as the magnetic cores are not a part of the presentinvention and any suitable prior art constructions may be used. Themagnetic cores may be of the type disclosed in the Begun application,Serial No. 688,738, filed August 6, 1946, which issued as Patent No.2,513,617 on July 4, 1950 'or in the Kornei application, Serial No.688,034 filed Aug. 2, 1946. Three cores may be used so that recordingand playback are effected on different cores, as'indicated for examplein the Begun application, Serial No. 713,963, filed Dec. 4, 1946. Threeseparate motors may be used to drive the two reels and the capstan, asuitable construction and control arrangement being disclosed in theabove-mentioned Dank application. If desired only two motors may be usedas. disclosed for example in the Williams application, Serial No.732,970 filed March 7, 1947. Suitable automatic limit switchingarrangements as disclosed in either of the last two mentionedapplications or in the Begun application, Serial No. 723,735 filedJanuary 23, 1947, are highly useful additions.

As described above each time the control assembly 81 is set for adifferent reeling operation the latch plate H0 is momentarily actuatedto the right hand position.

Accordin to one phase of the invention the apparatus may be arranged foreffecting a demagnetization of the magnetic cores each time the controlsare set for a different reeling operation. By this feature it ispossible to make sure the cores are effectively demagnetized immediatelybefore any transducing operation is comnetize the cores and clarify thesignal. I As illustrated in Fig. 1 one suitable arrange- -slide and thelatch plate menced. Furthermore if during a transducing operation suchas a playback, it is found that a background noise is interfering withthe signal, the transducing may be interrupted to demagment includes aswitch I40 positioned for actuation by right hand movement of the latchplate I I0. The switch I40 is shown connected in series with anotherswitch I42 to establish a relay actuating circuit for effecting thedemagnetization. The switch I42 may be arranged as shown for actuationwhenever the deflecting assembly 46 is ositioned for permitting therecord track to move in the guide path 3-IS, spaced and magneticallyisolated from the magnetic cores 31, 38.

A demagnetizing relay I 50 is shown as connected for operation by anexternal power supply which may be of the conventional cycle A. C. typethrough plug I54, On-Off switch I56 and the switches I40 and I42 inseries. The same power source may be used to actuate the amplifiers,oscillator and record track impelling means as by means of the leadsI58.

r The relay I50 operates a set of switches I5I, I52 and'l53 whichestablish the demagnetizing circuit proper. As shown the A. C. powersupply is connected to opposite terminals of a bridge network I60 byconductors III, I12, and switch I53 while the other terminals of thebridge are connected to the windings of cores 31, 38 by the switchesI5I, I52 and the ground connections shown. 1

The bridge I60 includes four'arms, two opposing arms containingresistors ISI of the type having the characteristic of increasing theirresistance with an increase in the current passed through them, such asincandescent lamps or varistors. The other arms contain ordinaryresistors I62 arranged to balance the bridge at the highest resistancevalues of the current sensitive resistors I6I during the demagnetizing.

In the arrangement shown in Fig. 1, operation of any ofthe push buttonsfrom an outer to an inner position causes the latch plate IIO tomomentarily close the switch I40 and if the switch I 42 is in the closedposition at the same time the relay I50 is actuated to close thedemagnetizing circuit.

One of the push buttons, as for example the Stop button may be labeledClear to indicate that transducing clarification by demagnetization maybe effected at any time by merely operating this button, even during atransducing operation. Since transducing is accompanied by opening ofthe'switch I42 provision may be made to insure sufficient overlapping inthe action of the two controlling switches I40 and I42 when theStopclear button is pressed during transducing-Lats by suitablyadjusting the free path of the deflecting before engaging the movableblades I43 and contact with the opposing contacts I 44 and respectivelyof these switches. Additionally a time delay means may be used to keepthe switch I40 closed for a short time after Forward push rod BI isunlatched. As shown the contact I44 of switch I40 may be biased towardthe left and arranged to have appreciable travel to the right under theurging of the movable blade I43 so that retraction of the blade I43 bywithdrawal of the latch plate IIO causes the contact I44 to fol low andstay in contact with the blade for an I45 and bringing them in assess:

Nfmay be provided with a dash-pot I90 to slow down its return to theleft, if desired. Only about 3 cycles of decaying current attenuationsare required for demagnetiz'ing so that for QO-cycle currents about /20of a secondis the minimum switch overlap required. However more cyclesmay be used if desired to provide more gradual decay and to allow forthe time necessary for the variable resistors to respond to the currentand to balance the bridge.

Many modifications of the invention may b made without changing itsbasic character. For example; instead of using the conventional A. C.powerline for erasing, alternating current supplied from any othersource such as the oscillator I6 may be used. Also, the demagnetizingsurge may be arranged to pass in series through the windings of thedifferent cores rather than in parallel. Other modifications include theprovision for the closing of the demagnetizing circuit to simultaneouslyalter the frequency of the oscillator so that the altered outputbypasses in sufficient amount through the condenser I9 to demagnetizeboth cores, thus simplifying the eonnections to the core windings.Furthermore instead of the push button assembly shown the controls mayinclude a rotary control rotatable to different positions forcontrolling the various reeling and transducing operations.- The switchI90 may, with this type of construction} be posi-- tioned for operationby a cam member rotatable with the control and having switch engagingcam surfaces for closing this switch as the control is rotated betweencontrol positions, as shown in the Begun application, Serial No.644,600, filed January 31, 1946, which issued as Patent No. 2,419,476onApril 22, 1947.

When using a portion of the apparatus as. a source of demagnetizingcurrent, suitable decay should be provided. For. example when anoscillator I9 of the vacuum tube type is used, output decay may bearranged by connections which open the 36+ power supply to theoscillator as the demagnetizing circuit is established. Enoughcapacitance may be provided on the oscillator 4 side of the powerconnection so that the B+ current in the oscillator does not suddenlystop but tapers of! slowly by discharge of the condenser over a timesufficient to permit the generation of the desired numbbr of decayingcycles. Any other "oscillation decaying means may be substituted.

Another. simple and highly effective arrangement for demagnetizing thecores is by discharging a condenser through the windings to producedamped discharge oscillations.

Fig. 2 illustrates one form of core'arrangement for demagnetizing bycondenser discharge. The core windings are here shown as connected tothe contacts I99, I94 of two double throw switches I9I, I92 of a switchassembly I90. The movable blades I99, I96 of the switches I9I, I92 arerespectively connected to one electrode of condensers I98 and I09, theother condenser electrodes being connected to the core windings by meansof the ground connections shown.

A suitable source of electrical energy 'for char in the condensers I88and I89 such as the D. C. power source indicated by the circled B+usually provided with any amplifier, one output terminal of which isgrounded, the other being connected to the contacts I91 and I99 of theswitches so that in the position of the switches shown in full lines thecondensers are charged, the tranducing functions of the cores areestablished by the connections between contacts I99, I94 respectivelyand the oscillator and amplifier. These transducing connections areshown via switch blades I99 and 200 respectively, which may be mountedfor actuation together with and may be part of the switch assembly I90.Other transducing switches may also be provided to vary the transducingconnections as shown for example by switch group. I20 in Fig. 1.

Upon movement of the switches I9I, I92, I99 and 200 from their full linepositions to the dashed line positions, the charged condensers I99 andI89 are disconnected from the charging source and connected across thewindings of the respective cores. Damped oscillatory discharge isaccordingly produced in each core winding circuit, the frequency of theoscillations being that at which the inductance and capacitance of thecircuit cause it to resonate. By selecting these frequency determiningcharacteristics, as for example the capacitances so that the frequencyof the oscillations-approximates that at which each circuit exhibits itshighest Q, the number of decaying discharge oscillations can beincreased to a maximum. Similarly by arranging for switches I99 and 290to disconnect the core windings from the transducing circuits at thetime of the discharges, any loading of the circuits such as may beeffected by a relatively low impedance oscillator or amplifier output todiminish the circuit Q will be avoided.

In the arrangement shown in Fig. 2 with cores that exhibit their highestQ at about 1000 cycles and with 0.1 microfarad condensers about 6 or 7decaying cycles of oscillations may be obtained with condenser chargingvoltages as low as 22 The discharge arrangement may be modified by usingonly one condenser and suitable switching to discharge through all thecore windings, in series. All the windings may then be designed toexhibit a high Q at the frequency of discharge.

If desired, the switches I99 and 200 may be eliminated so that the corewindings are directly connected to the transducing controls as in theconstruction of Fig. 1. As so modified the dischargesmay have a lessperfect demagnetizing action when brought about while the windings areconnected to the output of the oscillator or amplifier. However, therewill still be many highly eii'ectivedischarges produced while the recordpush button is retracted and only the windings of one core (core 98) areconnected to an amplifler input, which input has a relatively highimpedance and does not significantly load the circuit. Furthermore, theinward actuation of the Stop-clear button to demagnetize the heads willcause retraction of the other buttons so that the demagnetizingdischarges, when badly needed, are not excessively damped by any loads.

By way of further. modification the invention may be embodied in anapparatus in which the demagnetizing is produced by passing analternating current through the core windings and then attenuating thecurrent by means of a potentiometer connected across the alternatingcurrent supply.

Fig. 3 illustrates another form of the invention. In this embodiment therecord track 9| moving in the forward direction indicated by the arrow9IF has a plurality of alternate paths indicated by the dash-double-dotline 9IS, the dashtriple-dot line 9I--T and the dash-quadrupledot line 9IP along which it may be impelled to move along the guide rollers 29,29, a capstan roller 240 and erasing. recording and playback 75 magneticcores 91, 299 and 299 respectively.

Along the path il-S, the record track is shown as moving at somedistance from and out of magnetic linkage with the cores. A deflectingguide 40 which may be a fixed surface or a roller mounted on adeflecting arm 256 pivotally held as at 260 and having a cam followerportion or'toe 264 mounted for operation by a cam 200, to change theguide path betweeen the guides 25 so that the record track is urged intomagnetic linkage with the cores 31 and 235 as indicated by 3|T. Anotherdeflecting guide 49 is also shown on an arm 251 having a toe 265 foroperation by a cam 28! to change the record track guide path between theintermediate guide 26 and the capstan roller 240 so that the recordtrack is magnetically linked with the magnetic playback core 229 asindicated by Sl-P.

As indicated in the figure, the two cams 200, 28! may be arranged forsimultaneous actuation as by the rotation of a selector knob 290 held onthe cam shaft 204, and have projecting cam lobes 282, 203, 204 forengaging the toes 204, 265 and moving the arms against the action ofbiasing means, such as-the springs 218.

The magnetic cores are shown as having their windings connected to thevarious transducing elements, amplifiers, oscillator, etc, by means ofthe switch 220 and the switch assembly 22! under the control ofanother-cam 286 provided with cam lobes 221, 208 and 209and connectedfor actuation together with cams 280 and 20!.

The selector knob 290 is shown as rotatable into different controlpositions for rewinding, playback, fast forward reeling, recording ormonitoring as indicated by the legends. The knob may be held in place bythe latch gear 205 held on the cam shaft 204 and having teeth 20'!engaged by a latch member, such as the pivoted latch lever 2ll biased asby spring 2l2 toward a latching position defined. by a stop 2. The camshaft assembly may be biased as by the spring 2|! to urge the selectorin a counter-clockwise direction toward the rewind position. The nose 2|5 of the cam, however, is so shaped with respect to the teeth 201 as toresist such counter-clockwise rotation while permitting clockwiserotation of the teeth by manual operation of the selector knob 290, forexample. Manual rotation of the selector knob in a counter-clockwisedirection may be provided as by mounting the gear 205 and knob 290 sothat they are slidable along their axis of rotation, and the knob may beaxially moved to bring the cam 205 away from the plane of and out ofengagement with the latch lever. Other techniques for permitting manualcounter-clockwise control include the provision of conventional clutchmeans between the selector knob and the gear 205 so arranged thatwhenever rotatory power is transmitted from the selector knob to foractuation when the unreeling of the record track from the supply reel inearly completed.

As illustrated, the supply reel 20 is supported by a collar or table 225and is provided with one or more slots l9, located adjacent the floor llof the record track receiving space H as well as drive connections, suchas perforations, for receiving the top of the drive shaft l5 and a drivepin 220 extending up from the collar 226. Pivotally supported on thecollar is shown a sensing member 230 having at its inner portion asensing finger 232 positioned ,under an aperture 234 in the collar andbiased upwardly, as by the distribution of its own mass, toward the reel20. The reel slots l9 may be so related to the drive pin engagement thatthe mounting of a reel automatically positions a slot I9 over thesensing finger 232.

The outer end of the sensing member 230 is shown as provided with adepending lip 230, which when lowered by the upward movement of thesensing finger, will rotate with the collar 226 in a generallyhorizontal plane to strike the nearer end 242 of the limit member 225which may in turn be pivoted as at 244 and biased toward a stop 246.-When aportion of the inner turns of record track on the reel 20 obstructthe gear the knob first moves through a lost mo- 60 tion range duringwhich the clutch automatically moves the gear axially away from theplane of the latch or vice versa. As another alternative, the teeth 2"and latch nose 2 l 5 may be so shaped that the manual operation of theknob 290 can force the latch nose out against the latch bias to permitgear rotation, whereas the .urging of the shaft bias 2" is insufficientfor overriding thelatching action. I

The number of teeth "Ton the gear 205 need not be larger than one lessthan the number of control positions, as shown, a fixed stop 208 beingsuitable for defining the limiting positions.

The latch 2l0 is shown as connected by link 218 to a forward reelinglimit lever 225 arranged the upward movement of sensing finger 232, thelip 230 is retracted above and out of reach of the limit member 225. Thesensing finger may be positioned to move upwardly to cause tripping ofthe limit lever and cessation of unreeling with one or more anchoringturns of record track on the reel 20 so that a rewinding operation maybe immediately started without the necessity of securin the record trackend back on the supply reel 20.

The lip 236 and/or the limit member 225 may be provided with taperedsurfaces for permitting their en a ement during rewindingto lift the lipover the limit member end 242. As shown, this may be accomplished byshaping the lip with a slo ing face 246 on the edge which advancesduring rewinding, the other edge which advances during forward reelinghaving a vertical face 250.

The limit lever 225 and the latch 2l0' are so interlinked that-trippingof the lever 225 by the lip 23G disen ages the nose MS of latch 2l0 fromthe teeth 201 of gear 205 permitting the bias 2|! to return the shaft204 with all its cams to the limiting counterclockwise or rewindposition as defined by a suitable stop. such as stop 200.

Another cam 245 containing lobes 246 and 241 is also shown as providedon the shaft 204 to control the operation of the capstan roller 240. Inthe form illustrated, the cap tan roller is revolvably mounted as by thebearing 4| 9 on a lon itudinally slidable shaft 42L A flywheel 430 isalso revolvablv mounted as by bearing 43! held on a support 433 so thatits control portions are adiacent the surface of the capstan roller.

The lower end of shaft 42| is illustrated as coacting with a slide 435having a notch 438 so positioned that the lower end of shaft 42l, whichis downwardly biased, either dro s into the notch l1. tlonal drivingconnection between the roller Ill and flywheel I as by providing eitherwith a rubber-tired drive surface. 7

The slide 43! is shown as linked to. a cam follower I having a toe lurged as bybias 2 into engagement with the cam I. Under the actuation ofthis cam the slide ll! may be moved to permit the ofshaft "I to dropinto the notch I and lower the capstan roller Ill. The adjacent surfacesof the capstan'roller and the flywheel are so spaced that the droppingof the shaft I into the notch "I brings the capstan roller intofrictional driving engagement with' the flywheel. If desired, thisdriving engagement may be made by means of'a friction disc, such as afelt pad I, positioned intermediate the When frictionally engaged isdriven at the dasiredrecord track impelling speed. The lifting of theshaft I, as by engagement of the toe 3 with a low region of the the.capstan roller.

cam. 24!,lifts the capstan roller. out of engagement with the massiveflywheel to permit move ment of the record track along its guide pathwithout the burden of the high inertia, as for example, when a highspeed reeling is to be effected. The capstan motor is arranged to beenergized as required to impel the record track during a transducingoperation. An additional cam or circuit connection may be provided forclosing the energizing circuit to this motor at the proper positions orthe control elements.

As atlcally illustrated, another cam member ill on the cam shaft 204 hastwo can lobes 3", "I arranged for operating switches Ill and illconnected to establish power circuits to the reeling motors SI! and 3",respectively. These motors may be directly connected to the shafts I!and It on which the supply and take-up reels are respectively mounted,as indicated. Power to these motors may be supplied by the ordinaryelectric power lines. through the plug-in connector III, On-Oif switchI2! and switch "I.

The 8+ D. 0. power supply circuit to the oscillator It as well as theamplifiers may also be arranged for control by switch 323, as indicatedby connectors I, and the rectifler III fed from the output of theswitch.

The switch III is also shown as coacting with the control assembly I"illustrated as including a pair of push rods 82!, '28 labeled Start andStop, respectively, slidably held in a frame 38.

and interlinked by a latch plate It! laterally slidable with respect tothe push rods and biased -as by spring "I to engage cam projections ason these rods. Both rods are biased outwardly as by springs 33! and theswitch 823 is biased to open position andarranged to be closed by inwardactuation of the Start push rod 32. to establish the motor circuits tothe switches I and Ill.

l2 in the desired setting. This causes the actuation of either thesupply reel motor III for rewinding or the take-up reel motor forforward reellng. Inward actuation of theStop push rod 128 .unlatches theStart push rod and permits switch "I to open and stop the reeling.

In the construction illustrated, the take-up reel I II is also providedwith a reeling limit control for terminating a reeling operation whenthe record track is completely unreeled from the reel. As shown, thisreel may be mounted in a manner similar to, that shown for the supplyreel 20 and the limit control may be of a generally similar nature. Forreasons more fully set forth below the take-up reel limit controlt shownas including a sensing finger 2 which projects from the take-up reelcollar 221 and enters the inner regions of a slot is and engages theinnernmst turn or record track through a passageway I in the floor ll ofthe record track reeiving space ll The sensing finger 2 is shown as heldon a sensing member "6 siidably positioned on the collar as by means ofa lug 350 and a passageway :52 in the collar. The sensing member "8 mayI be outwardly biased as by spring I" or the centrifugal force developedduring rotation which for rewinding may be of a relatively highspeed.

In the general plane of the outer end III of the sensing member I ismounted the end 3'" of a limit member I in such position as not toobstruct the end I" as the slide 3 in the inward position in which it isheld by the engagement of the finger I and the inner turn of recordtrack rotates with the collar 221. I

The limit member is shown as connected by link 360 with an unlatchinglever It! pivotally mounted to engage the latch plate "I as by means ofthe pin 3: whe during a reeling the Start push rod is in its inwardposition and the limit member 854 is tripped by the end I" of thesensing slide 6, the latch plate is automatically moved to the right topermit the push rod bias to retract the Start rod and allow the switchIN to open to stop the reeling According to the invention, means isprovided to automatically demagnetize the magneto cores when the rewindlimit control ,is actuated. One

- form of such arrangement is shown in the figure as including a switchassembly "0 containing a group of switches 4M, 8 and l automaticallyactuated by the unlatching lever I when the rewind limit member l istripped.

Switches 6 and I are shown in this construction as connecting thewindings of the diiferent cores with the high frequency output of theoscillator 18. The demagnetizing arrangement is so constructed as toimpart decay characteristics to the output and improve its demagnetizingproperties. In the form shown, the assembly "I is so disposed thatactuation of the lever "l flrst closes the switches "I. "I and I andthen, after a lag which may be only momentary. unlatches the Start rod3" and opens the switch 323. The closing of switches 4M, 4"

and 08 supplies alternating currents of large peak magnitudes tothe'core windings to insure substantial saturation of the cores. The,opening of switch .23 interrupts the 3+ power supply and the inherent,capactance-on the oscilla tor sldeof the switch I. then discharges itsstored energy so that the oscillator output dies I out more gradually.An additional capacitor I To start a reeling operation, the Start pushrod is actuatedafter the selector knob'is positioned ll may be shuntedacross the D. 0. power line, as shown, to improve the decaycharacteristics it with the above apparatus. the necessary minimum ofabout three decaying cycles of demag-- netizing alternations is readilyobtained and effective core demagnetizations result.

Attention is called to the fact that the appa- I transducings has beenpreceded by a series of successive non-transducing reeling operations.In the past such a series of reeling operations by themselves would inmany cases build up the background noise level to such an extent as torequire a special demagnetizing operation. This difficulty isexceptionally serious when the magnetic cores are used with: recordtracks of the kind in which the magnetic medium is a stratum of finelydivided permanently magnetizable material, such as described in theKornei applications, Serial Nos. 685,092 and 685,093, filed July 20,1946, the latter application having been abandoned.

It is believed that the large number of nonmagnetic gaps present betweenthe particles of such magneti strata provide a great many magneticpolesh ghly sensitive to extremely slight magnetic variations at thecores. The overall magnetic response of an elemental length of such arecord track bridging the non-magnetic gap of a magnetic core is thestatistical average of the magnetic condition of the individual poleslying the bridging length and is easily changed by a variation in themagnetic condition of only a small portion of the poles. With magneticrecord tracks, such as steel wire or tape having an unbroken magneticmedium, the difliculties though not, as frequent are stillobjectionable. In any case, the apparatus of the invention is quiteimmune to the almost unpredictable disturbances previously experiencedwith transducers and particularly frequent when used with record tracksin which the magnetizable medium is finely divided. The most usuallynoticed prior .art disturbance is the increase in background noise levelto a point which in severe cases almost completely masks the signal. Insome instances the magnetic cores had to be removed from the transducerand demagnetized by being placed in and taken out of a magnetic field.

According to the invention, no special manual operation is needed toprevent the prior art disturbances and an unskilled layman can operate ahome type recorder with very high quality results. The amount and costof materials necessary to provide the automatic demagnetization is quitesmall as can be seen from the above exemplifications.

The invention is applicable to transducers using either A. C. or D. C.bias and used with record tracks of any form, such as tapes or wire-likefilaments. The other features of the transducers shown are not a part ofthe present invention and any reeling and/or control arrangements may beu ed. For example, where high frequency currents, such as those providedby the conventional cscillator, are not used for demagnetizing thecores, the erasing core may desirably be of the type using ordinarypower frequencies fed through its windings, so that the oscillator maybe simplified to produce only small bias currents and I not the largecurrents necessary for erasing or demagnetizing. Erasing cores usingpower line frequencies are disclosed and claimed in the copending Begunapplication, Serial No. 723,374, filed January 21, 1947.

Additionally. the neutralizing means for feeding demagnetizing currentto t i e cores, as shown in Fig. 3, need not be actuate by limit reelingcontrols coacting with the reels but may be arranged for operation bylimit controls mounted in the record track guide path, as shown forexample in the copending Begun application, Serial No. 723,735, filedJanuary 23, 1947, or in the copending Dank application, Serial No.690,878, filed August 16, 1946. Also the limit control or demagnetizingfeed control may be actuated by special physical propertiesof the recordtrack itself, there being provided in the guide path a special controlwhich is sensitive to the special properties. For example, anelectrically conductive strip may be secured near the end of anelectrically non-conducting record track and a pair of contacts may beso disposed as to close a circuit when the record track moving initsguide path brings the conductive strip across them. This as well asother arrangements are also described in the above-mentioned Dankapplication.

According to another modification of the invention, a magnetictransducing apparatus may be arranged for operation by controls moresuitable for actuation from a distance. This feature may be provided bya stepping magnet or solenoid such as those used to rotate a shaft insequential small steps in a manner conventionally used for steppingswitches. A construction embodying this feature may be in generalsimilar to that of Fig. 3 modified by permitting the camshaft 204 or itsvarious cams to rotate completely around the shaft axis. The latch gear20,) together with the latch 2"], stop 208 and bias 2|2 may be omittedand the stepping solenoid linked to the cams so that with each actuationof the solenoid windings the cams on the cam shaft are turned through anare that brings them to the next operating position.

The operating positions may be distributed in uniformly spaced relationaround the 360 degrees of cam rotation in an endless sequence each cycleof which may have the following steps in order Fast forward RewindPlayback Stop Record so that by merely closing the solenoid circuit'asfrom a long two wire control cable having one end connected to a powersupply as well as to the solenoid windings and the other end providedwith a switch, and desired transducing operation may be effected.Repeated actuation of the solenoid will rotate the cams so that. theysuccessively establish the operations of the sequence from Fast forwardto Stop. after which the next relay actuation again establishes Fastforward operation.

With such a construction the monitoring operation is not required andonly two magnetic cores are needed. The reeling limit control membersmay be arranged to also close the stepping circult and automaticallyadvance the cams to the next operating position of the endless sequence.For example when the reeling of a recording operation is completed, theforward reeling limit member will close a switch sending a power pulsethrough the solenoid windings and placing the apparatus in rewindingcondition and beginning the rewinding. Near the end of the re assaseswinding, the rewind limit member will repeat the power pulse therebystopping the reeling. The

rewind step may be arranged to become effective before the record trackis completely removed from the, take-up reel 1i so that furthersequential transducing may be performed by remote control withoutrethreading the record track.

In the control sequence described above, at the end of a playbackoperation the apparatus will automatically be placed in condition forrecording even though the forward reeling is completed. The forwardlimit control member such as lever 225 may be arranged so that therepeated engagement by the sensing lever Ill-when moved into theengaging position causes the lever 22! to send repeated pulses throughthe windings of the stepping relay each pulse thereby steps the cams tothe next position, As so arranged, the first turn of the supply reel atthe end of a playback will automatically set the apparatus forrecording. Since the recording involves additional forward reeling, thesupply reel will make another revolution andthe forward limit controlwill be again operated to send a second pulse through the relay toautomatically advance the cams another step to the rewind position. Theinitiation of rewinding may then be arranged to promptly stop theforward reeling to prevent the supply reel from making a third completerevolution in the forward direction so that the rewinding willautomatically continue without a further step of th! cams which thirdstep would cause stopping.

The control cable may be provided with an indicator mechanism to showthe operating position of the apparatus to the control operator. Theindicator mechanism may include a second stepping solenoid in the remotecontrol end of the control cables operated in parallel with thecamstepping solenoid and arranged to rotate an indicator cardso as toexpose the proper card 16 inserted where desired, after the fast forwardstep for example.

In those constructions in which the core neutralizing currents are fedto the cores under the control of a sensing member responsive to thepresence of recordtrack in the guide path between the recordtrackholdingreels, suitable arrangement may be provided to render the demagnetizingmeans inoperative at all times except when a rewinding operationiscompleted. For example, an interlocking control, such as a switch, maybe arranged to be operated by the's'etting of the apparatus controlsinto a rewinding position and only operate the demagnetizing means whenthe interlock switch is closed and the sensing member calls for coredemagnetizlng as by indicating the:h absence ofrecord Win the rewindguide pa In the forms of the invention described above in connectionwith Fig. 3, all the record trackguide paths may be in ma netic contactwith one or more of the cores and may even be identical. For example thepole faces of the cores may be burnished so as to minimize frictionagainst the moving record track and the non-transducing reelingoperation} such as rewinding and fast forward reeling may be arranged totake place with the record track contacting these cores and preferablywith the pressing parts restricted so as not to hold the record trackfirmly against the cores. As so. arranged the demagnetizing of the coresunder the predetermined conditions described in connection with Fig. 3is just as highly effective.

portion bearing a suitable legend through a viewing aperture.Alternatively a flexible shaft connected to the cam shaft 204 may bearranged directly rotate the indicator card.

The Start-stop switch assembly 324 is not needed in the steppingmodification and may be eliminated to' simplify the controls. Thedemagnetizing switch mechanism 400 may be directly operated by therewind limit control but.

one of the switches of this group may be'omitted if only two magneticcores are used, as indicated above.

The remainder of the sequentially-controllable apparatus may be quitesimilar to that shown in \Fig. 3 including capstan control cam 245,motor control cam 30 i, record track guiding cam 280 and transducingcontrol cam 286. The cam lobes, of course, will have to be redistributedto fit the step positions. Furthermore the amplifying and transducingcircuits and switches may be modifled for use with only two magneticcores in the manner indicated in Fig. 1 for example, provision beingmade if desired to open the transducing circuits during operations notinvolving transducing, as when-the record track is being rewound. Ofcourse stepping sequences other than that described above may be usedwithin the scope of the-invention. Some or all of the steps'may beinverted in order. The fast forward step may follow the playback or therecord step and the playback step may follow the record step.Furthermore amonitoring step may be inserted together with themonitoring features of the construction of Fig. 3. Additional Stop stepsmaybe- Additionally the capstan drives shown in connection with Figs. 1and 3 may be modified without affecting the basic principles of theinvention. For example the capstan may be arranged to be automaticallyor manually disconnected from the record track during non-transducingreeling operations, as shown in the above-mentioned Dank and Begunapplications.

In this application by a recitation of a transducing operation isintendeda recording operation or a playback operation or an erasingoperation or any combination of two orjmore of such operations.

.The principles of the invention explained in connection with specificexempliflcations thereof will suggest tothose skilled in the art manyother applications and modifications of the same. It is accordinglydesired that the appended claims be construed broadly, and that theyshall not be limited to the specific details shown and dezctribed inconnection with exempliflcations there- I claim: i

1. In a magnetic record transducing apparatus for magnetically recordingsignals of the audio frequency range with a superposed alternating biasfield of at least twice higher frequency on a magnetizable recordingmedium moving past transducer head means along a transducing path andfor reproducing recorded-signals therefrom; a recording mediumcomprising an elongated flexible record track having a bonded stratum ofpermanently magnetizable powder particles; trans ducer head meansincluding at least one set of transducer windings and a loop-likemagnetic core structure extending essentially along one side of theadjacent moving record track and magnetically interlinked with saidwindings and with successive elements of the moving record track: guidemeans including control means seset to cause said apparatusto performeither a recording operation or a reproducing operation; means includingsaid transducer head means and said recording medium whereby saidapparatus will record signals with a superposed bias field current whichis substantially below a level sufficient to saturate said corestructure; and normally inefiective neutralizing means having a sup-\ply circuit including at least some windings interlinked with said corestructure and actuable to magnetize said core structure with at leastthree current waves of alternating current, starting with a wave portionof relatively high intensity sufficient to saturate said core structure,and ending with a wave portion of low intensity effective in reducingthe magnetization of said core structure to substantially zero; saidcontrol means including activating means whereby said control means willstart a reproducing operation in response to the'actuation of saidsetting means and actuate said neutralizing means to supply said currentwaves to said windings of said core structure and effect substantialneutralization of said core structure.

2. In a magnetic record transducing apparatus as claimed in claim 1,said control means also including means for delaying movement of .saidtrack along said transducing path past said head means until substantialcompletion of said neutralization.

3. In a magnetic record transducing apparatus as claimed in claim 1,said activating means being also operative in response to actuation ofsaid setting means to start a recording operation for actuation of saidneutralizing means and effect said neutralization of said corestructure.

4. In a magnetic record transducing apparatus as claimed in claim 3,said control means also including means for delaying movement of saidtrack along said transducing path past said head means until substantialcompletion of said neutralization.

57 In a magnetic record transducing apparatus as claimed in claim 1, thesetting means of said control means being also selectively settable to astop setting, whereby said control means may be selectively set to stop,said operations, said activating means being also operative in responseto actuattion of said setting means either to start a recordingoperation or to stop said operations for actuation of said activatingmeans and effect said neutralization of said core structure.

6. In a magnetic record transducing apparatus as claimed in claim 5,said control means also including means for delaying movement of saidtrack along said transducing path past said head means until substantialcompletion of said neutralization.

'7. In a magnetic record transducing apparatus as claimed in claim 1,further characterized by means to impel said track at a desiredtransducing speed during said transducing operations, and by means toimpel said track at a higher nontransducing speed along anon-transducing path in which all elements of the moving track aremaintained spaced away from said head means to remain at all timn= eticlinkage be selectively set to cause said apparatus to perform either anon-transducing operation, or to stop said operations, said activatingmeans being also operative in response to actuation of said settingmeans to start either a recording operation or a non-transducingoperation, or to stop said operations, for actuation of said activatingmeans and to efiect said neutralization of said core structure.

8. In a magnetic record transducing apparatus.

as claimed in claim '7, said control means also including means fordelaying movement of said track along said transducing path past saidhead means until substantial completion of said neutralization.

'9. In a magneticrecord transducing apparatus as claimed in clafin 1,further characterized by means to impel said track at a desiredtransducing speed during said transducing operations, and by means toimpel said track at a higher nontransducing speed along -anon-transducing path in which all-elements of the-moving track aremaintained spaced away from said head means to remain at all times outof magnetic linkage with said core structure, the setting means of saidcontrol means being also selectively settable either to anon-transducing setting, or a stop setting, whereby said control meansmay also be selectively set to cause said apparatus to perform either anon-transducing operation, or to stop said operations, said activatingmeans being also operative in response to actuation of said settingmeans to start either a recording operation or to stop said operations,for actuation of said activating means and effect said neutralization ofsaid core structure, and means preventing activation of saidneutralizing means when said setting means is set to a non-transducingsetting and performing a non-transducing operation.

in which all elements of the moving track are maintained spaced awayfrom said head means to remain at all times out of magnetic linkage withsaid core structure; the setting means of said control means being alsoselectively settable to a non-transducing setting, whereby said controlmeans may be selectively set to cause said apparatus to perform anon-transducing operation; said apparatus including a pair of reels onwhich the major part of the track is held in coiled form with anintermediate portion of the track extending between the reels formovement in forward direction along said transducing path when the trackis reeled from the storing reel onto the take-up reel during atransducing operation; said control means also including limit meansactuated upon termination of a forward reeling operation to rewind saidtrack along said non-transducimz with rm'rn the said storing reel; andlimit assasos means operative in response to termination of a rewindingoperation for actuating said neutralizing means to eiiect saidneutralization 0! said core structure.

12. In a magnetic record transducing apparatus as claimed in claim 1,further characterized by means to impel said track at a desiredtransducing speed during said transducing operations, and by means toimpel said track at a higher non-transducing speed along anon-transducing path in which all elements of the moving track aremaintained spaced away from said head means to remain at all times outof magnetic linkage with said core structure; the setting means of saidcontrol means being also selectiveLv 'settable either to anon-transducing setting, or a stop setting, whereby said control meansmay also be selectively set to cause said apparatus to'perform either anon-transducing operation, or to stop said operations; said activatingmeans being also operative in response to actuation of said settingmeans to start either a recording operation or to stop said operationsfor actuation of said activating means and eflect said neutralization orsaid core structure; said apparatus including apair of reels on whichthe major part of the track is held in coiled form with an intermediateportion of the track extending between the reels for movement in forwarddirection along said transducing path when the track is reeled from thestoring reel onto the take-up reel during a transducing operation; saidcontrol means also including limit means actuated upon termination of aforward reeling operation to rewind said track along saidnon-transducing path onto the said storing reel; and limit meansoperative in response to termination of a rewinding operation foractuating said neutralizing mean :0 eflect said neutralization 01' saidcore strucure.

13. In a magnetic record transducing apparatus as claimed in claim 12,and means preventing activation 01. said neutralizing means when saidcontrol means is actuated to a non-transducing setting.

emu Josnrn BEGUN.

name-noes mm The following references are of record in the file of thispatent:

, UNITED s'm'ras Pam-rs Number Germany June 20, 1940

